Transmission system with flexible frame structure

ABSTRACT

In a transmission system a transmitter ( 2 ) comprises a frame assembler ( 8 ) for assembling frames with a header and a payload portion. The header ( 22 - 1 ) comprises a validity indicator (SSSSS) to indicate the validity of a sequence of ATM cells in the frame. By using values of the validity indicator (SSSSS) different from the value of the validity indicator for valid ATM cells, it becomes possible to transport data in a format different from a sequence of ATM cells.

[0001] The invention is related to a transmission system comprising atransmitter being coupled to at least one receiver via a transmissionmedium, the transmitter comprising frame assembling means for assemblingframes comprising a control portion and a sequence of ATM cells, thetransmitter further comprises validity indication inserting means forinserting an validity indication in the control portion regarding thevalidity of the sequence of ATM cells, the receiver comprising framedisassembling means for extracting the ATM cells from the frames if thevalidity indication indicates valid ATM cells.

[0002] The invention is also related to a transmitter, a receiver, atransmission method and a signal.

[0003] A transmission system according to the present invention is knownfrom DAVIC 1.1 Specification, Part 8, Revision 3.0.

[0004] In the DAVIC specifications (Digital Audio VIdeo Council), it istried to standardise a digital enhanced broadcast chain. These standardscover the complete chain from the content provider via the serviceprovider to the end user. Aspects covered by DAVIC are e.g. videocoding, security, channel coding, modulation and frame structures.

[0005] In DAVIC it is proposed to use MPEG-2 transport stream multiplexpackets, in which the 187 bytes payload part carry a frame comprising acontrol portion and a sequence of ATM cells. The control portion cancarry several items such as the priority of a packet, an error flag, anindication of the first frame of a multiframe sequence. It can alsocarry an indication that the current sequence of ATM cells carried inthe frame has to be discarded at the receiver because the ATM cells areintroduced as stuff information to fill the transmission channel.

[0006] A problem of the known system is that it is not suitable fortransporting data in other formats than ATM cells. Such formats can e.g.be STM, which requires fixed length data words having a repetition rateof 125 μs, or variable length data formats such as Ethernet packets orIP (Internet Protocol) packets.

[0007] The object of the present invention is to provide a transmissionsystem according to the preamble which allows the transport of varioustypes of data formats, without loosing the possibility of transportingATM packets as described above.

[0008] Therefor the transmission system according to the presentinvention is characterised in that the frame assembling means are alsoarranged for inserting data different from a sequence of ATM cells intothe frame, in that the validity indication insertion means are arrangedfor introducing an identification of the data different from a sequenceof ATM cells by a different value of the validity indicator, and in thatthe frame disassembling means are arranged for extracting the datadifferent from a sequence of ATM cells from the frames, if the validityindicator indicates data different from a sequence of ATM cells.

[0009] The present invention is based on the idea that it is possible touse the validity indication which is used for indicating the validity ofthe present sequence of ATM cells to indicate other types of dataformats. By doing so, the transport of ATM cells according to the priorart is not affected at all, but only the possibility to add other typesof data formats is created. In the DAVIC standard the control portioncomprises 5 bits “ 11110” indicating the validity of the ATM cells.Consequently other values of these 5 bits can be used for indicatingother data formats.

[0010] An embodiment of the present invention is characterised in thatthe data different from a sequence of ATM cells comprises packets withan identification portion indicating of at least one property of saidpackets.

[0011] By specifying at least one property of a cell in anidentification portion, it is obtained that such cells easily can beidentified and processed accordingly.

[0012] A further embodiment of the invention is characterised in thatthe at least one property comprises the length of the packet and in thatthe frame comprises a data portion indicating the position of thebeginning of a new packet in the frame.

[0013] By specifying the length of each packet in the frame, and byspecifying the beginning of one packet in the frame, the position ofeach packet in the frame can easily be determined. This allows an easyextraction of the packets from the frame.

[0014] A further embodiment of the invention is characterised in thatthe length of the packet is an integer multiple of an elementary dataunit larger than one byte.

[0015] By introducing these elementary data units, the length of thepackets can have only a restricted number of values, reducing the numberof bits required to encode the length of the packets.

[0016] The present invention will now be explained with reference to thedrawings. Herein shows:

[0017]FIG. 1, a transmission system according to the invention;

[0018]FIG. 2, the constitution of a basic frame consisting of two MPEGtransport stream frames;

[0019]FIG. 3, a packet comprising an ATM cell with an identifier byteaccording to the IEEE802.14 proposal;

[0020]FIG. 4, a packet comprising a variable length cell;

[0021]FIG. 5, a packet comprising an STM cell;

[0022]FIG. 6, a block diagram of the controller 18 for a frame structurebeing able to carry the data formats according to FIG. 3, FIG. 4 andFIG. 5.

[0023] In the transmission system according to FIG. 1, an ATM signal isapplied to a first input of a multiplexer 4 in a transmitter 2. An STMsignal is applied to a second input of the multiplexer 4, and a sequenceof variable length cells is applied to a third input of the multiplexer4. A first output of a controller 6 is connected to a control input ofthe multiplexer 4. An output of the multiplexer 4 is connected to aninput of the frame assembling means further to be referred to as frameassembler 8.

[0024] A second output signal of the controller 6, carrying an outputsignal V-ATM, is connected to a first input of the validity indicationinserting means, further to be referred to as validity indicationinserter 10. A third output of the controller 6, carrying an outputsignal V-STM, is connected to a second input of the validity indicationinserter 10. A fourth output of the controller 6, carrying an outputsignal V-VL, is connected to a third input of the validity indicationinserter 10.

[0025] An output of the validity indication inserter 10 is connected toa second input of the frame assembler 8. The output of the frameassembler 8 is coupled to the output of the transmitter 2.

[0026] The output of the transmitter 2 is coupled to the input of areceiver 14 via a transmission medium 12. The input of the receiver 14is connected to an input of a controller 18, and to an input of theframe disassembling means, being here a frame disassembler 16. An outputof the controller 18 is connected to a second input of the framedisassembler 16. At a first output of the frame disassembler 16 anoutput signal in ATM format is available. At a second output of theframe disassembler 16 an output signal in STM format is available, andat a third output of the frame disassembler 16 an output signalcomprising variable length packets is available.

[0027] The transmission system according to FIG. 1 is arranged fortransmitting data in ATM format and STM format. It also supports thetransmission of variable length packets such as Ethernet packets orTCP/IP packets. Elementary units of the different types of data arepacked in so-called Protocol Data Units (PDU's).

[0028] The controller 6 submits to the multiplexer 4 a control signalindicating which of its input signals is to be passed to the output ofthe multiplexer 4. The combination of the controller 6 and themultiplexer 4 are arranged to change the selection of the sourcerapidly, introducing the possibility of generating an output stream inwhich ATM cells, STM data and variable length packets are interleaved.

[0029] The controller 6 provides information about the validity of thesignals at the inputs of the multiplexer, in order to enable thevalidity indication inserter 10 to introduce a validity indication inthe frame to be constructed by the frame assembler 8. By using thevalidity indication the receiver is enabled to distinguish idle packetsfrom packets carrying payload data. By using an identification otherthan already prescribed in the DAVIC specification, it becomes possibleto identify other types of data formats.

[0030] The frame assembler 8 constructs frames comprising the outputdata of the multiplexer 4 and the validity indication. The frames ofdata are transmitted to the receiver 12 via the transmission medium 12.

[0031] In the receiver 14, the controller determines the beginning of aframe, and it determines from the validity indication whether the inputsignal carries a stream of ATM cells or that the input signal carries adifferent type of signal. This information is used to control thedisassembler 16 which extracts the different data formats from the frameand outputs them at corresponding outputs of the receiver 14. It isobserved that it is possible that a receiver is arranged for onlyreceiving one type of data. In such a case only a signal is present atthe output of the received if said type of data is present in the frameat the input of the receiver 14.

[0032] The signal according to FIG. 2 comprises a sequence of 2 standardMPEG transport multiplex packets. A frame according to the presentinvention is carried by the 187 payload bytes of two subsequent MPEGtransport stream packets. The first of these MPEG transport multiplexpackets comprises a synchronisation signal 20 of one byte, the payload22 of 187 bytes and an error control portion 24 of sixteen bytes. Thesecond of these MPEG transport multiplex packets comprises asynchronisation signal 24 of one byte, the payload 26 of 187 bytes andan error control portion 28 of sixteen bytes. The synchronisation signalis a fixed 8 bit field with binary value “01000111”, and it is used forframe synchronisation. The frame comprises four control bytes 22-1 (CTRL0), 26-1 (CTRL 1), 26-5 (CTRL 2) and 22-6 (CTRL 3), and a plurality ofelementary data units 22-2 . . . 22-5, 26-2 . . . 26-5 also calledslots, to carry the user data. These slots comprise an integer multipleof bytes. It is not necessary that after the CTRL 0 byte a new slotstarts, but it is possible that a slot from a previous frame iscontinued. This leads to an increased efficiency of the frame, becauseno unused space is present in the frame. The CTRL 0 byte indicates thatthe present packet is the first of the sequence of two MPEG transportstream packets. The value of CRTL 0 is E1PSSSSSb where the E, P, and Sbits are defined below. The CTRL 1 byte indicates that the currentpacket is the second of a two packet sequence. Its value is E0PSSSSSbwhere E, P, and S bits are defined below.

[0033] The E bit is a 1 bit error flag. When set to “1”, it indicatesthat at least 1 uncorrectable bit error exists in the associated 187byte payload. This bit may be set to “1” by entities in the transportlayer. When set to “1”, this bit shall not be reset to “0” unless thebit value(s) in error have been corrected.

[0034] The P bit is a 1 bit priority flag. When set to “1”, it indicatesthat the associated packet has a higher priority than the payload ofwhich the priority flag is set to “0”.

[0035] The 5 bit SSSSS field is the validity indicator. If its value isequal to binary “11110” it means that a valid stream of ATM cellsaccording the DAVIC standard is within the payload. In such a case noslots are used. The first MPEG transport stream packet comprises 3 ATMcells of 53 bytes and 27 bytes of a fourth ATM cell. The second MPEGtransport stream packet comprises the 26 remaining bytes of the fourthpacket and 3 further ATM cells. In this case the CTRL 3 byte is notpresent.

[0036] If the string SSSSS has another value e.g. “01011”, it means thata signal differing from the above mentioned ATM stream is present withinthe payload 22 and 26. It is possible to indicate with the string SSSSSwhich type of signal is within the payload, but it is also possible toindicate with the string SSSSS only that the payload carries a differentformat than the DAVIC ATM stream. The latter opens the possibility thatdifferent types of data are interleaved within the payload. In such acase, this data has to carry identification information itself.

[0037] The CTRL 2 byte is reserved. It will be defined for carriage ofOperation, Administration, and Maintenance information (OAM).

[0038] The CTRL 3 byte indicates the position of the first byte of a newProtocol Data Unit (PDU) in the second MPEG transport packet. Each newPDU starts wit a new timeslot. By using this information, thedisassembler 16 can easily extract the PDU from the frame.

[0039]FIG. 3 shows a first type of PDU being a sequence of ATM cellsaccording to a draft for the (not yet announced) IEEE 802.14 standard.This sequence differs from the standardised DAVIC sequence of ATM cellsby an identification portion being here a one byte identifier 40preceding each ATM cell. The identifier byte 40 has three fields beingdefined according to the table below: Field Usage Size Format ID Type ofPDU (set = 00) 2 bit Encryption Key Even/Odd Encryption key identifier 1bit Reserved Reserved for expansion of Format ID (set = 0) 5 bits

[0040] The format ID 40-1 field indicated the type of PDU. This field ispresent in all PDU's. For the ATM PDU these bits are set to “00”. Theencryption key identifier bit 40-2 is used for indicating a switchbetween two sets of encryption keys in order to increase security. Theportion 40-3 comprising the final 5 bits of the identifier 40 arereserved for introducing future format ID's.

[0041]FIG. 4 shows a second type of PDU being a variable length cellPDU. This type of cell has a identifier 44 with four fields. The fieldshave the meaning according to the following table. Field Usage SizeFormat ID Type of PDU (set = 1) 1 bit Encryption Key Even/Odd Encryptionkey identifier 1 bit Sequence Sequence identifier for fragmentation/ 2bits reassembly 10 = First data fragment 00 = Fragment within a VL cell01 = Last data fragment 11 = Start and end of VL cell are both withinthis fragment. Size Number of slots that follow for the same 4 bitsfragment

[0042] The format ID field 44-1 is a bit indicating the type of PDU. Itis set to the value “1 ”. The encryption key identifier 44-2 has thesame meaning as already discussed in reference with FIG. 3. The sequenceidentifier 44-3 is present to deal with variable length data unitslarger than 143 bytes. If the sequence identifier 44-3 has the binaryvalue “10”, it means that the current fragment is the first fragmentfrom a larger VL cell. If the sequence identifier 44-3 has the binaryvalue “00”, it means that the current fragment is a fragment within theVL cell and that at least one fragment will follow. If the sequenceidentifier 44-2 has the binary value “01”, it means that the currentfragment is the final segment of the VL cell. If the sequence identifier44-3 has the binary value “11 ”, it means that the current fragment isthe only fragment of the VL cell. The sequence field is used to extractthe complete VL packet from a plurality of VL PDU's. The size identifier44-4 consists of four bits representing the number of 9 bytes slots thatfollow for the same PDU.

[0043]FIG. 5 shows a third type of PDU being intended for carrying STMdata. STM is used to support fixed bitrate 64 Kbps connection, oftenintended for telephony or ISDN applications. The STM PDU comprises a onebyte header 46, and an 8 byte STM cell 48. Consequently the STM PDUfills exactly one slot. The header 46 comprises three fields 46-1, 46-2and 46-3 according to the table below. Field Usage Size Format ID Typeof PDU (set = 01) 2 bit Encryption Key Even/Odd Encryption keyidentifier 1 bit Stream Identifier STM stream identifier. The NT canfilter the 5 bits incoming S_PDUs on value of this field. Each streamcontains data of up to 8 calls.

[0044] The format ID 46-1 with a value of “01” indicates the presence ofan STM PDU. There are various options to implement STM in thedownstream.

[0045] A first way of transporting the STM data is to use PDUs with thesize of a single slot. Again the header byte contains the cell typeinformation, being a cell type identifier and an STM Stream Identifier.A single byte per slot per 64 kbps connection will be allocated, wherethe slots are scheduled 125 Ts apart from each other. In case only one64 kbps connection is active, this would create an overhead of 7 bytesper 125 Ts in a 30 Mbps stream which is about 1.5%. This method supportsaddressing of up to 256 simultaneous active calls per downstreamcarrier.

[0046] An alternative way to transport STM which is used in the PDUaccording to FIG. 5, is to introduce a stream identifier of 5 bits foridentifying the actual STM stream for which the data in the STM cell isintended. Due to the size of the stream identifier, the number ofsimultaneous active calls is 32. The STM cells comprises 8 subsequentbytes from one STM stream. This leads to an additional delay of 8×125μs=1 ms, due to the buffering of 8 bytes from the ATM stream.

[0047] In the control unit 18 according to FIG. 6, the input isconnected to an input of a frame synchroniser 51, to a control byteselector 52 and to a PDU header selector 56. An output of the framesynchroniser 51 is connected to an input of a reset input of a framebyte counter 50. An output of the frame byte counter 50, carrying thenumber of the present byte in the frame, is connected to an input of alogic unit 58 and to an input of the control byte selector 52.

[0048] A first output of the control byte selector 52, carrying thevalidity indicator SSSSS for the ATM cells is connected to the logicunit 58. A second output of the control byte selector 52 , carrying areset signal, is connected to a PDU-byte counter 54. An output of thePDU-byte counter 54, carrying the number of the present byte in thepresent PDU, is connected to the logic unit 58 and to the PDU headerselector 56. A first output of the PDU header selector, carrying thenumber of bytes in the present PDU is connected to an input of thePDU-byte counter 54. A second output of the PDU header selector 56,carrying a signal representing the type of the present PDU, is connectedto the logic unit 58. The output of the logic unit 58 constitutes theoutput of the controller 18.

[0049] The frame byte counter 50 is a counter being able to count from 0to 186. It outputs the number of the current byte in the frame. Theframe synchroniser 51 determined the beginning of each frame of 187bytes. The frame synchroniser 51 issues at the beginning of each frame areset pulse to the frame byte counter 50 to reset it. This causes theframe byte counter to be synchronised with the frame at the input of thecontrol unit 18.

[0050] The control byte selector 52 is arranged for extracting thecontrol bytes CRTL 0, CRTL 1, CRTL 2 and CRTL 3 from the input stream.The control byte selector 52 is arranged to select the bytes at theinput as control bytes ifs the frame byte counter 50 has the value 0 orthe value 156. If the first bit in a control byte found at a position 0of the frame byte counter has a value “1 ”, said control byte is a CTRL0 byte. Its value is stored, and the value of the bits SSSSS is passedto the logic unit 58. If the sequence SSSSS is equal to “11110”, theframe comprises a sequence of ATM cells according to the DAVIC standard,and the logic unit 58 issues a command to the frame disassembler 16(FIG. 1) to pass all payload to the ATM output. If the sequence differsfrom “11110”, the frames comprises PDU based data.

[0051] The control byte present at the position 186 of the frame bytecounter 50 can be a CRTL 2 or CRTL 3 byte. If the control byte in thesame frame at position 0 of the frame byte counter 50 was a CRTL 0 byte,the byte at position 186 in a CRTL 3 byte. Otherwise the byte atposition 186 is a CRTL 2 byte.

[0052] In the case a CRTL 3 byte is present, its content indicating thebyte number on which the first new PDU in the next frame (or part of theframe) starts, is stored for later use.

[0053] The PDU byte counter 54 is arranged for counting PDU bytes. It isarranged as a down counter which starts from a preset value and itcounts down to 0. The PDU byte counter 54 is decremented only if a PDUbyte is present in the payload. It is not decremented during thepresence of CRTL 0, CRTL 1, CRTL 2 or CRTL 3 bytes. The PDU byte counteris reset to “0” by the control byte selector 52 if the value of theframe byte counter corresponds to the value of the previous CRTL 3 byte.This reset indicates the beginning of a new PDU. The PDU header selector56 selects from the input signal the present byte if the PDU bytecounter 54 has a value “0”. The PDU header selector determines the typeof PDU and the length of the PDU from the information present in theheader.

[0054] An ATM cell PDU can be recognised by the value “00” of the firsttwo bits of the PDU header. The length of such an ATM PDU is 54 bytes.The cell type is passed to the logic unit 58, enabling to route theinput signal via the disassembler 16 (FIG. 1) to the ATM output. Thelogic unit 58 is arranged for presenting a read command to thedisassembler 16 only if the value of the PDU counter has changed and thevalue of the PDU byte counter is not equal to 0. This is done to preventthe CRTL bytes and the PDU header being passed to one of the outputs ofthe receiver. If a CRTL byte is at the input, the PDU byte counter isnot advanced, and consequently the CRTL byte is not passed to the outputof the receiver. If a PDU header is present at the input, the PDU bytecounter has a value of 0, and hence the PDU header is not passed to theoutput of the receiver. The number of PDU bytes after the first PDU byte(#PDU bytes) is 53. This number is loaded in the programmable PDU bytecounter 54. The PDU byte counter is decremented each time a byte of datais read from the input. The PDU byte counter will have the value zeroafter 53 bytes have been read from the input. Subsequently the PDUheader selector reads the header of the next PDU.

[0055] A VL PDU can be recognised by the value “1 ” of the first bit inthe PDU header. The number of PDU bytes (#PDU) after the first PDU byteis determined from the length indicator L represented by the four lastbits of the PDU header according to: #PDU bytes =8+L*9. The type of PDUis also passed to the logic unit 58.

[0056] An STM PDU can be recognised by the value “01” of the first twobytes of the PDU header. The length of such a PDU (including header) is9 bytes, resulting in a value of 8 for #PDU bytes. The type of PDU andthe stream identifier are passed to the disassembler 16. The latter isused to identify the STM stream the bytes in the present PDU belong to.This information is used for routing the STM signals correctly to theirfinal destination.

1. Transmission system comprising a transmitter being coupled to atleast one receiver via a transmission medium, the transmitter comprisingframe assembling means for assembling frames comprising a controlportion and a sequence of ATM cells, the transmitter further comprisesvalidity indication inserting means for inserting an validity indicationin the control portion regarding the validity of the sequence of ATMcells, the receiver comprising frame disassembling means for extractingthe ATM cells from the frames if the validity indication indicates validATM cells, characterised in that the frame assembling means are alsoarranged for inserting data different from a sequence of ATM cells intothe frame, in that the validity indication insertion means are arrangedfor introducing an identification of the data different from a sequenceof ATM cells by a different value of the validity indicator, and in thatthe frame disassembling means are arranged for extracting the datadifferent from a sequence of ATM cells from the frames, if the validityindicator indicates data different from a sequence of ATM cells. 2.Transmission system according to claim 1 , characterised in that thedata different from a sequence of ATM cells comprises packets with anidentification portion indicating of at least one property of saidpackets.
 3. Transmission system according to claim 2 , characterised inthat the at least one property comprises the length of the packet and inthat the frame comprises a data portion indicating the position of thebeginning of a new packet in the frame.
 4. Transmission system accordingto claim 3 , characterised in that the length of the packet is aninteger multiple of an elementary data unit larger than one byte. 5.Transmitter comprising frame assembling means for assembling framescomprising a control portion and a sequence of ATM cells, thetransmitter further comprises validity indication inserting means forinserting an validity indication in the control portion regarding thevalidity of the ATM cells, characterised in that the frame assemblingmeans are also arranged for inserting data different from a sequence ofATM cells into the frame, in that the validity indication insertionmeans are arranged for introducing an identification of the datadifferent from a sequence of ATM cells by a different value of thevalidity indicator.
 6. Transmitter according to claim 5 , characterisedin that the data different from a sequence of ATM cells comprisespackets with an identification portion indicating of at least oneproperty of said packets.
 7. Receiver for receiving frames comprising acontrol portion and a sequence of ATM cells, said control portioncomprising a validity indication regarding the validity of the ATMcells, the receiver comprising frame disassembling means for extractingthe ATM cells from the frames if the validity indication indicates validATM cells, characterised in that the frame can also carry data differentfrom a sequence of ATM cells into the frame, in that the validityindicator comprises an identification of the data different from asequence of ATM cells, and in that the frame disassembling means arearranged for extracting the data different from a sequence of ATM cellsfrom the frames, if the validity indication indicates data differentfrom a sequence of ATM cells.
 8. Receiver according to claim 7 ,characterised in that the data different from a sequence of ATM cellscomprises packets with an identification portion indicating of at leastone property of said packets.
 9. Transmission method comprisingassembling frames with a control portion and a sequence of ATM cells,the control portion comprising a validity indication regarding thevalidity of the ATM cells, characterised in that the frame can alsocomprise data different from a sequence of ATM cells, and in that thevalidity indication comprises an identification of the data differentfrom a of sequence of ATM cells.
 10. Signal comprising frames with acontrol portion and a sequence of ATM cells, the control portioncomprising a validity indication regarding the validity of the ATMcells, characterised in that the frame can also comprise data differentfrom a sequence of ATM cells, and in that the validity indicationcomprises an identification of the data different from a sequence of ATMcells.